Many enterprises are replacing their existing CENTREX (Central Exchange) or PBX (Private Branch Exchange) based telephony systems with VoIP (Voice over IP (Internet Protocol)) systems based upon SIP (Session Initiation Protocol) signalling systems. Such systems utilize existing IP infrastructure based upon LAN (Local Area Network) and WLAN (Wireless Local Area Network) technologies.
At the same time, mobile phones that use Cellular networks such as GSM (Global System for Mobile Communications), CDMA (Code Division Multiple Access) and UMTS (Universal Mobile Telecommunications System), to conduct communications with others inside and outside the enterprise network, are being enhanced to provide SIP based communications, including VoIP, over WLAN access, allowing these devices to provide mobile telephony communication capability when within the enterprise network as well as when away from the enterprise network.
Although systems based on SIP are being widely deployed as the common signalling protocol for VoIP telephony, there is a lack of standardization in the way SIP is utilized. Also, the protocol continues to be developed further with continued introduction of new extensions to SIP. This has led to different versions of SIP with significant differences in the way they are used. These differences include the usage of different SIP headers for conveying the same information or to trigger the same action, different call scenarios (different message sequences) for the same call setup or call feature, the use of proprietary or pre-standard SIP headers, and the use of new extensions to the SIP standards.
These differences create a problem for manufacturers of mobile devices such as mobile phones. Mobile phones manufactured with a version of SIP software that implements an early version of SIP may be sold and attempted to be used on a network that requires a version of SIP software that implements a later SIP standard. It may not be economically viable for vendors of such mobile devices to produce different SIP software for every potentially deployed network SIP usage.
As more versions of SIP are in use it becomes more important for network providers to provide mappings between the various versions. Moreover, it becomes important to be able to map SIP to other communication protocols such as Hyper Text Transfer Protocol ‘HTTP’. A protocol mapping might be updated each time a new version of SIP is introduced, or whenever a mapping is to be provided for another existing protocol. However, this might result in the protocol mapping being updated very frequently, which can be inconvenient and impractical.